Offshore pipeline recovery

ABSTRACT

An offshore pipeline is recovered from the bottom of a body of water by introducing gas into one end of the line and pumping liquids from the other end of the pipeline until a section of the line at least sufficiently long to reach to the surface of the body of water has been substantially voided of liquids, and thereafter lifting the end of the pipeline to the water&#39;&#39;s surface. This procedure substantially reduces the cost of pipeline recovery operations and has other advantages over methods used in the past.

United States Patent 1191 Matthews, Jr;

[ OFFSHORE PIPELINE RECOVERY [75] Inventor: Jamie F. Matthews, Jr.,Houston,

Tex.

[73] Assignee: Esso Production Research Company,

Houston, Tex.

22 Filed: June14, 1972 21 Appl. NO.: 262,815

521' U.S. c1. 6l/72.3, 114/50 51 Int. Cl. B63c 7/12,-F161 1/00 .[58]Field of Search 61/72.3, 72.1, 72.4;

[56] References Cited UNITED STATES PATENTS 2,602,300 7/1952 Collins61/72.4

2,363,488 11/1944 Bartlett ..ll4/50 [4 Dec. 11, 1973 3,631,830 1/1972Welch 114/50 3,525,226 8/1970 McCarron ..61/72.3

Primary Examiner-Jacob Shapiro AttorneyJames A. Reilly et a1.

[57] ABSTRACT 1 An Offshore pipeline is recovered from the bottom of abody of water by introducing gas into one end of the line and pumpingliquids from the other end of the pipeline until a section of the lineat least sufficiently long to reach to the surface of the body of waterhas been substantially voided of liquids, and thereafter lifting the endof the pipeline to the waters surface. This procedure substantiallyreduces the cost of pipeline recovery operations and has otheradvantages over methods used in the past.

20 Claims, 4 Drawing Figures PATENIED DEC! 1 I915 SHEET 10F 2 OFFSHOREPIPELINE RECOVERY BACKGROUND OF THE INVENTION 1. Field of the InventionThis invention relates to the recovery of offshore pipelines and isparticularly concerned with a method for recovering the end of a damagedline from the bottom of a body of water.

2. Description of'the Prior Art Offshore pipelaying operations aregenerally carrie out by means of a laybarge or similar vessel whichmoves along the pipeline route. As the pipe is lowered into place fromthe stern of the moving vessel, it bends under its own weight andassumes a generally S-shaped configuration between the vessel and thebottom of the body of water. The amount .of bendingwhich takes placedepends upon the weight and dimensions of the pipe, the depth of ,thewater, the amount of tension maintained in the pipe, and other factors.Excessive bending will result in buckling or kinking of the pipe andhence a pipe discharge ramp or stinger which extends into the waterbehind the vessel and supports the pipe is normally used to limit theradius of curvature. Despite the precautions normally taken, however,buckling and flooding of the pipeline may occur, particularly if thevessel is caught in a sudden storm and the mooring system, tensioningequipment or stinger fails for some reason. This generally requires thatthe end of the pipeline be raised to the waters surface and repaired.

The recovery of an offshore pipeline after it has buckled duringpipelaying operations is generally a time consuming and expensiveprocedure. Before the line can be raised to the surface, it is usuallynecessary to void the line to prevent overstressing of the pipe andavoid additional buckling. This is normally done by first having a divercut off the damaged portion of the pipe with an underwater electricarc-oxygen cutting torch, burn holes through the pipe wall, and theninstall a bar or long bolt through the holes to block the end of thepipe. After the bar or bolt has been installed, apipeline pig isinserted into the line at an accessible point onshore or at an offshoreplatform to which the pipeline is connected. Compressed air is thenintroduced into the pipeline from the shore or platform to move the pigforward and displace water from the line. After the pig has reached theblocked end of the pipe and the line has thus been voided of liquids,cables are attached to the line and the end is raised to the surface ofthe water. Once this has been done, the end of the pipe is cut off and anew section is welded in place to permit the resumption of pipelayingoperations. Methods generally similar to these are used for the recoveryof pipelines which have been damaged after being put into service.

A major disadvantage of the methods referred to above is that theyrequire that the pipeline pig be run through the entire line from theshore installation or platform to the point at which the damage exists.This may necessitate pigging of the line over a distance of severalmiles and may require the use of high pressure compressors for longperiods of time. It may be difficult to provide suitable compressors atthe required locations. If the pipeline to be recovered is onecontaining oil or a similar material, the pigging operation may resultin discharge of the contents of the line into the water and may causeserious pollution problems. As a result of these and relateddifficulties, such methods often leave much to be desired.

SUMMARY OF THE INVENTION This invention provides an improvement inpipeline recovery methods which largely eliminates the problems referredto above. The improved method involves the introduction of air or gasinto one end of a marine pipeline after the damaged portion has been cutoff on bottom and the pumping of liquids from the other end of the lineby means of a pump which is lowered into the pipeline through a riser ona platform to which the line is attached or is connected to the end. ofthe line on bottom. The pumping of liquids from the pipe and thesimultaneous introduction of gas is continued until a section of thepipeline at least sufficiently long to extend to the waters surface hasbeen substantially voided of liquids. Thereafter, cables can be attachedand the line can be lifted to the surface under tension without seriousbuckling problems. Similar procedures may be employed to permit therecovery and repair of damaged lines containing oil and other liquids.

The manner in which the method of the invention is carried out willdepend in part upon the particular pipeline to be recovered. The pumpused to withdraw liquids from the line will normally be installed at thelower end of the line. In the case of a line which slopes upwardly froman offshore platform or similar structure so that the end to berecovered is at a higher elevation than the base of the platform orother structure, for example, a pump will normally be lowered into thepipeline through the platform riser. An air line extending to acompressor or the atmosphere will be connected to the other end topermit the introduction of air or gas. If the sea floor slopesnon-uniformly, a pigging device may be inserted into the line. Thesimultaneous introduction of air or gas and withdrawal of water from asection of the pipeline sufficiently long to reach the surface willpermit lifting of the end of the line under tension with little dangerof buckling. In the case of a pipeline which slopes downwardly from aplatform or shore installation so that the end to be recovered is at alower elevation than the rest of the line, on the other hand, analternate technique will normally be employed. In this case, the pumpwill be connected to the submerged end of the line and air or gas willbe admitted to the other end at the platform or shore: installation. Theuse of a pigging device may be advantageous. Water or other liquidspresent in the line will then be pumped out until essentially the entirepipeline is filled with air or gas. This procedure eliminates any dangerthat liquids will flow into the suspended portionof the line after it israised to the waters surface. In either case, the method permitssubstantial savings in recovering the end of the submerged pipeline,avoids the necessity of injecting large volumes of high pressure air orgas to displace water or other liquids present in the line, reduces theloss of oil or other materials that might cause serious pollutionproblems if discharged into the body of water in the conventionalmanner, permits recovery and repair without the necessity of havingdivers on bottom for long periods of time, and reduces the time requiredfor recovery and repair operations.

The equipment employed in carrying out the method may be of conventionaldesign and is normally available from commercial sources. Any of avariety of different electrically, hydraulically, or mechanicallyactuated submersible pumps that can be lowered at the end of a string ofpipe or hose into a pipeline riser or connected to the submerged end ofa pipeline by welding or mechanical means may be employed for pumpingwater from the line. Air or gas can be introduced in some cases bysimply venting the line at the surface and in other cases byconnecting abull plug or similar fitting equipped with an air or gas line to thesubmerged end of the pipeline. Where a pigging device is to be used, anordinary pipeline pig or an inflatable sphere of rubber or similarmaterial may be used. In lieu of this, specially designed equipmentwhich can be quickly attached to the end of the line and actuated fromthe surface to effect a closure and provide an inlet for air or gas canalso be employed. Regardless of the particular equipment selected, themethod of the invention has numerous advantages over pipeline recoverymethods which have been employed in the past. As a result of theseadvantages, the method has wide application and may permit the carryingout of recovery operations that would not be feasible with methodsavailable heretofore.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 in the drawing illustrates useof the method of the invention for the recovery of one end of anunderwater pipeline which extends alone the ocean floor from a riser onan offshore platform;

FIG. 2 depicts a later stage in the operation of FIG.

FIG. 3 shows an operation similar to that of FIGS. 1 and 2 for therecovery of one end of a pipeline which has not yet been connected to ariser on an offshore platform; and

FIG. 4 illustrates use of the method for the recovery of a pipeline laidinto deep water from an onshore installation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 in the drawing depictsan offshore pipeline recovery operation carried out in accordance withthe invention. As shown in FIG. 1, underwater pipeline 11 has been laidalong the bottom 12 of a body of water 13 from an offshore platform orsimilar structure 14. In the particular installation shown, the end ofthe pipeline adjacent the platform has been connected to the lower endof a pipeline riser 15 which extends upwardly 'to a point above thewaters surface 16 and is held in place on the platform by means ofclamps or similar members 17. The type and configuration of the riserwill depend upon the particular structure from which the pipeline hasbeen laid and upon other factors. In some cases, for example, the risermay have been formed from line pipe by bending the pipe to produce alarge radius curve near the base of the structure and will thus be anintegral part of the pipeline; while in other cases a separate riser maybe connected to the end of the pipeline andinstalled on the structure.In still other instances, the riser may be one of the structural membersof the platform or may pass upwardly through such a structural memberand may include a flexible joint designed to accomodate limited movementof the structure and riser. The method of the invention is thusapplicable to any ofa variety of different pipeline installations.

As illustrated in FIG. 1, the laying of pipeline 11 has been interruptedat a point several miles from platform 14. The interruption may haveoccured as a result of equipment malfunctioning during a severe stormwhich resulted in dropping, buckling and flooding of the pipe. Althoughnot shown in the drawing, the pipeline will normally be encased in acorrosion coating and cement jacket which serve to protect the pipe andgive it the required weight. These have been removed from the end of theline. The cutting off of the damaged portion and the removal of thecoating and jacket can be accomplished by divers or in very deep watercan be carried out with a submersible vessel fitted with manipulators. Arubber ball or similar pigging element has been inserted in the end ofthe pipeline and a bull plug or other closure 18 fitted with an airinlet and suitable packing has been welded or mechanically connected inplace on the end of the pipe over the ball. Air line 19 and liftingcable 20 extend upwardly from the closure 18 to the laybarge or similarvessel 21 from which the recovery operation is to be carried out. Theair line is secured aboard the barge and vented to the atmosphere; whilethe lifting cable extends from a winch or crane 22. The mooring linesemployed to hold the barge in position over the end of the pipeline arenot shown in the drawing.

After the closure 18 and air line 19 have been installed as describedabove, a pump which does not appear in the drawing is lowered intoposition in pipeline riser 15 by means of a string of flexible tubing,pipe, steel reinforced hose, or similar conduit 23. The pipe extendsupwardly above the upper end of the riser and is fitted with a bend 24for the discharge of liquids pumped upwardly through the tubing string.The pump and tubing are supported in the riser by means of a line fromcrane 25. The pump employed may be electrically, hydraulically ormechanically actuated. In cases where the riser bend precludes loweringof the pump to the bottom of the riser, a flexible tail pipesufficiently long to extend downwardly into the pipeline itself may beattached to the bottom of the pump to serve as a liquid intake line.Following installation of the pump and pipe or tubing string, liquidsare pumped upwardly through the tubing to the surface. If the liquidcontained in the pipeline is sea water, the discharge will normally bedisposed of over the side as shown in FIG. 1. If, on the other hand, thepipeline is one that has been damaged in service and the liquidcontained therein is oil or a similar material, the effluent from thetubing string will be directed into tanks on the platform or into abarge'moored nearby. As liquid is pumped out of the pipeline,atmospheric pressure exerted against the pigging element by air enteringthrough air line 19 propels the pigging element forward through thepipeline. In a 24-inch pipeline, the total force available to move thepigging element is in excess of 6,000 pounds and hence the use of highpressure air or gas to propel the pigging element is unnecessary. Thiseliminates the need of a high pressure, high capacity compressor aboardbarge 21. In some cases, however, it may be advantageous to both pumpliquid from the line in front of the pigging element and injectcompressed air or gas into the line behind the element in order toexpedite the operation. If this is done, a compressor of much lowerdischarge pressure and capacity than would otherwise be required may beused.

The pumping of liquid from the pipeline of FIG. 1 as described above iscontinued until a section of the line long enough to extend to thesurface of the water has been substantially voided of liquid. Since theweight of the pipe and the depth of the water are known, the length ofthe section from which the liquid must be removed can be readilycalculated. By measuring the volume of liquid discharged from thepipeline at platform 14, the arrival of the pigging device at thenecessary point in the pipeline can be determined. Thereafter, liftingcable can be transferred to a winch and used to lift the end of thepipeline to the surface of the water. Sufficient tension should bemaintained on the line during the lifting operations to prevent the linefrom buckling. The tension required, which will be much less than if theline were filled with liquid, can be readily calculated. Multiple cablesspaced at intervals along the section of the line to be raised can alsobe used for lifting purposes. FIG. 2 in the drawing shows the pipelineafter the end has been lifted to the surface of the water andtransferred to a stinger 26 at the stern of laybarge 21. With the end ofthe line in this position, closure 18 can be removed from the end and anew length of pipe can be weldedin place to permit the resumption ofnormal pipelaying operations.

FIG. 3 in the drawing illustrates an operation which is similar to thatdescribed in connection with FIGS. 1 and 2 but differs in that thepipeline has not yet been connected in place to a riser at offshorestructure 30. As indicated in the drawing, the end of the pipeline 31has been secured in place on the ocean floor 32 by means of a cable 33extending to the base of the platform. A submersible pump 34 powered byan electrical cable'or hydraulic line 35 extending upwardly to the deckof the platform has been connected to the end of the pipe. A check valve36 or a hydraulically actuated valve powered by means of a hydraulicline extending to the surface may be connected to the discharge side ofthe pump if necessary to prevent the backflow of water into the pipelinewhen the pump is not in operation. Depending upon the particular type ofpump employed, such a valve may or may not be needed.

The recovery of the other end of pipeline 31 in FIG. 3 of the drawing iscarried out in much the same manner as was the recovery of pipeline 11in FIGS. 1 and 2. As illustrated in FIG. 3, a bull plug or similarclosure 38 fitted with an air inlet has been attached to the end of theline. Air line 39 extends upwardly to the surface of the water 40 and issecured aboard laybarge or similar vessel 41. Hoisting cable 42 isconnected to the end of the line and to crane 43 aboard the barge. Thepigging device inserted in the end of the line prior to installation ofthe bull plug or similar closure does not appear in the drawing. Afterthe required volume of water has been pumped from the pipeline and thepigging device has moved through a section of the pipeline at least aslong as the length required to reach the waters surface, pump 34 is shutdown and valve 36 is closed if necessary to prevent backflow into theline. The voided line can then be picked up by means of the hoistingcable and pipelaying operations can thereafter be resumed. The pump atthe other end of the line can be recovered when the riser is installedand the pipeline is connected into place. V

It will be noted that in the operations shown in FIGS. 1, 2 and 3 of thedrawing the ocean floor slopes upwardly from the offshore platform sothat the end of the pipeline to be recovered is at a higher level thanthe end adjacent the platform. Under these circumstances, it is notnecessary to dewater the: entire pipeline. All that is required is thatsufficient water be withdrawn to prevent liquid from filling thesuspended portion of the line when the closure is removed from the endaboard the laybarge. In some cases, however, the bottom contour may besuch that removal of the closure is apt to result in backflow into thesuspended portion of the line. In such cases, it may sometimes bedesirable to pump as much liquid as possible from the line andsimultaneously inject high pressure air or gas from a source aboard thelaybarge or similar vessel until the pigging device has traversed theentire line and substantially all of the liquids have been displaced.This displacement of liquids by both pumping out the line in front ofthe pigging device and injecting gas under pressure behind the devicenormally permits significant savings over the use of high pressure airor gas alone.

FIG. 4 in the drawing illustrates an alternate procedure which may-beemployed where backflow into the suspended portion of the pipeline mightotherwise present difficulties. As shown in FIG. 4, pipeline has beenlaid along the bottom 51 of a body of water 52 from onshore installation53. The pipelaying operation has been interrupted in deep water becauseof equipment malfunctioning during a storm or other events whichresulted in dropping, buckling and flooding of the line. The pipe willagain normally be provided with a corrosion coating and a cement jacketfor weighting purposes and may be buried in part. The damaged portion ofthe line has been cut off on bottom by a diver using an electricarc-oxygen torch or similar equip ment. It may be lifted to the surfaceand taken aboard laybarge 54 if desired. An electrically orhydraulically actuated pump 55 powered by means of line 56 has beenlowered into place on cable 57 from crane 58 and connected to the end ofthe line. This may be done by the diver or, in very deep water, asubmersible vessel fitted with manipulators may be used for both cuttingoff the end of the pipe and welding or mechanically connecting the pumpin place. A check valve 59 designed to prevent backflow will normally beattached to the discharge side of the pump but in some cases the use ofsuch a valve may not be necessary. If desired, a hose or string offlexible tubing 60 may be connected to the outlet of the check valve ordischarge side of the pump to conduct the discharged liquids to thesurface. This is not normally necessary where the pipeline contains onlywater. I

Following connection of the pump to the end of the pipeline on bottom asdescribed above, the other end onshore is vented to the atmosphere or,if the pipeline contains oil, connected to a low pressure source ofnatural gas or other gas which will not support combustion. A piggingdevice may be inserted in the open end of the line if desired but thisis usually unnecessary. The pumping of water from the line is thencommenced. After essentially all of the water has been pumped out andthe line has filled with air, as indicated by the change in load on thepump or by means of a signal actuated by arrival of the pigging deviceat the pump, pumping can be terminated. The airor gas-filled line cannow be lifted to the surface for repair without serious danger ofbuckling or related. problems. Since the entire line is essentially freeof liquids, no backflow will occur on removal of the pump and checkvalve at the surface.

The method illustrated in FIG. 4 can also be used for the recovery ofdamaged lines containing oil or other liquids that cannot be dischargedfrom the pump into the surrounding body of water. In an operation ofthis later type, reinforced hose, a string of flexible tubing, or asimilar conduit 60 will be connected to the check valve on the end ofthe pump so that the liquid can be conducted to the surface anddischarged into tanks aboard the laybarge or an auxiliary barge moorednearby. After the pipeline has been substantially voided of liquids, theflexible hose or tubing can be raised to the surface as the end of thepipeline is lifted. This minimizes pollution of the water and permitsrecovery of the contents of the line for later sale or reuse.

It will be apparent from the foregoing that the invention provides aneffective and relatively inexpensive method for the recovery ofpipelines which can be used regardless of the contour of the bottom andthe position of one end of the line with respect to the other. Thepumping of liquids from one end of the line while admitting air or gasat the other as described above permits substantial savings in pipelinerecovery operations, minimizes the loss of oil or other liquidscontained in lines damaged after they have been placed in service,avoids the necessity for using large volumes of high pressure gas duringrecovery operations, and has other advantages over recovery methodsproposed in the past.

I claim:

1. A method for recovering a submerged end of a liquid-filled pipelinefrom the bottom of a body of water which comprises introducing a gasinto one end of said pipeline and pumping liquid from the other end ofsaid pipeline until liquid has been voided from a section of saidpipeline adjacent said submerged end sufficiently long to reach at leastto the surface of said body of water and thereafter lifting saidsubmerged end to the waters surface.

2. A method as defined by claim 1 wherein said gas is introduced byventing said pipeline to the atmosphere.

3. A method as defined by claim 1 wherein said gas is introduced at apressure in excess of atmospheric pressure.

4. A method as defined by claim 1 wherein said liquid is pumped fromsaid pipeline by means of a pump lowered into said pipeline through apipeline riser.

5. A method as defined by claim 1 wherein said liquid is withdrawn bymeans of a pump connected to an end of said pipeline on bottom.

6. A method as defined by claim 1 wherein the introduction of said gasand the pumping out of said liquid are continued until said pipeline hasbeen substantially voided of liquid.

7. A method as defined by claim 1 wherein said gas is air and saidliquid is water.

8. A method as defined by claim 1 wherein a pipeline pigging device isinserted into said pipeline prior to the introduction of said gas andthe pumping out of said liquid.

9. A method for recovering a submerged end of a liquid-filled pipelinewhich slopes upwardly to said submerged end from an offshore structurewhich comprises inserting a pigging device in said submerged end of saidpipeline, introducing a gas into said submerged end and pumping liquidfrom the other end of said pipeline until said pigging device hastraversed a section of said pipeline sufficiently long to extend atleast to the waters surface, and thereafter lifting said submerged endof said pipeline to the surface of the water.

10. A method as defined by claim 9 wherein said liquid is pumped fromsaid other end of said pipeline by lowering a submersible pump into apipeline riser to which said other end is connected and thereafteractuating said pump.

11. A method as defined by claim 10 wherein said submersible pump islowered in said riser near the end of a conduit and liquid is pumpedthrough said conduit to the waters surface.

12. A method as defined by claim 9 wherein said liquid is pumped fromsaid other end of said pipeline by connecting a submersible pump to saidother end on bottom and thereafter actuating said pump.

13. A method for recovering a submerged end of a liquid-filled pipelinewhich slopes downwardly along the bottom of a body of water to saidsubmerged end which comprises connecting a submersible pump to saidsubmerged end of said pipeline, pumping liquid from said submerged endof the pipeline and simultaneously introducing gas into the other end ofthe pipeline until said pipeline has been substantially voided ofliquids, and thereafter lifting said submerged end of said pipeline tothe waters surface.

14. A method as defined by claim 13 wherein said gas is introduced intosaid other end of said pipeline by venting said other end to theatmosphere.

15. A method as defined by claim 13 wherein said gas is introduced intosaid other end of said pipeline at a pressure in excess of atmosphericpressure.

16. A method as defined by claim 13 wherein said liquid is oil and saidoil is pumped from said submerged end of the pipeline through a conduitextending to the waters surface.

17. A method as defined by claim 13 wherein said liquid is oil and saidgas is one which does not support combustion.

18. A method as defined by claim 13 wherein a pigging device is insertedin said other end of said pipeline prior to the introduction of said gasin said other end.

19. A method for recovering an end of a flooded pipeline from the bottomof a body of water on which one end of the pipeline is at a higherelevation than the other end of the pipeline which comprises pumpingwater from the end of said pipeline which is at the lower elevation,simultaneously introducing gas into the end of the pipeline which is atthe higher elevation, continuing the pumping out of said water and theintroduction of said gas until a section at one end of said pipelinesufficiently long to extend at least to the waters surface has beenfilled with gas, and thereafter lifting the end of said pipelineadjacent said section filled with gas to the surface of said body ofwater.

20. A method as defined by claim 19 wherein the pumping of water fromsaid end of said pipeline at the lower elevation and the introduction ofgas into said end of said pipeline at the higher elevation are continueduntil said pipeline has been substantially voided of water and said endof said pipeline at said lower elevation is thereafter lifted to thesurface of said body of water.

UNITED STATES PATENT OFFICE @ETEFICATE @F CORRECTION Patent No.3,777,499 Dated December 11, 1973 Inventor(s) Jamie F. Matthews, Jr.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 7; line 51, delete "Withdrawn" and insert "pumped. from saidpipeline.

Signed and sealed this 16th day of April 197A.

(SEAL) Attest:

EDWARD MJ'LETGHER,J1. O. MARSHALL DANN Attesting Officer CommissionerpfPatents FORM P0-1050 (1 -6 USCQMM-DC 60376-P59 k UrSr GOVERNMENTPRINTING OF FlCE r I969 0-4164!

1. A method for recovering a submerged end of a liquid-filled pipelinefrom the bottom of a body of water which comprises introducing a gasinto one end of said pipeline and pumping liquid from the other end ofsaid pipeline until liquid has been voided from a section of saidpipeline adjacent said submerged end sufficiently long to reach at leastto the surface of said body of water and thereafter lifting saidsubmerged end to the water''s surface.
 2. A method as defined by claim 1wherein said gas is introduced by venting said pipeline to theatmosphere.
 3. A method as defined by claim 1 wherein said gas Isintroduced at a pressure in excess of atmospheric pressure.
 4. A methodas defined by claim 1 wherein said liquid is pumped from said pipelineby means of a pump lowered into said pipeline through a pipeline riser.5. A method as defined by claim 1 wherein said liquid is withdrawn bymeans of a pump connected to an end of said pipeline on bottom.
 6. Amethod as defined by claim 1 wherein the introduction of said gas andthe pumping out of said liquid are continued until said pipeline hasbeen substantially voided of liquid.
 7. A method as defined by claim 1wherein said gas is air and said liquid is water.
 8. A method as definedby claim 1 wherein a pipeline pigging device is inserted into saidpipeline prior to the introduction of said gas and the pumping out ofsaid liquid.
 9. A method for recovering a submerged end of aliquid-filled pipeline which slopes upwardly to said submerged end froman offshore structure which comprises inserting a pigging device in saidsubmerged end of said pipeline, introducing a gas into said submergedend and pumping liquid from the other end of said pipeline until saidpigging device has traversed a section of said pipeline sufficientlylong to extend at least to the water''s surface, and thereafter liftingsaid submerged end of said pipeline to the surface of the water.
 10. Amethod as defined by claim 9 wherein said liquid is pumped from saidother end of said pipeline by lowering a submersible pump into apipeline riser to which said other end is connected and thereafteractuating said pump.
 11. A method as defined by claim 10 wherein saidsubmersible pump is lowered in said riser near the end of a conduit andliquid is pumped through said conduit to the water''s surface.
 12. Amethod as defined by claim 9 wherein said liquid is pumped from saidother end of said pipeline by connecting a submersible pump to saidother end on bottom and thereafter actuating said pump.
 13. A method forrecovering a submerged end of a liquid-filled pipeline which slopesdownwardly along the bottom of a body of water to said submerged endwhich comprises connecting a submersible pump to said submerged end ofsaid pipeline, pumping liquid from said submerged end of the pipelineand simultaneously introducing gas into the other end of the pipelineuntil said pipeline has been substantially voided of liquids, andthereafter lifting said submerged end of said pipeline to the water''ssurface.
 14. A method as defined by claim 13 wherein said gas isintroduced into said other end of said pipeline by venting said otherend to the atmosphere.
 15. A method as defined by claim 13 wherein saidgas is introduced into said other end of said pipeline at a pressure inexcess of atmospheric pressure.
 16. A method as defined by claim 13wherein said liquid is oil and said oil is pumped from said submergedend of the pipeline through a conduit extending to the water''s surface.17. A method as defined by claim 13 wherein said liquid is oil and saidgas is one which does not support combustion.
 18. A method as defined byclaim 13 wherein a pigging device is inserted in said other end of saidpipeline prior to the introduction of said gas in said other end.
 19. Amethod for recovering an end of a flooded pipeline from the bottom of abody of water on which one end of the pipeline is at a higher elevationthan the other end of the pipeline which comprises pumping water fromthe end of said pipeline which is at the lower elevation, simultaneouslyintroducing gas into the end of the pipeline which is at the higherelevation, continuing the pumping out of said water and the introductionof said gas until a section at one end of said pipeline sufficientlylong to extend at least to the water''s surface has been filled withgas, and thereafter lifting the end of said pipeline adjacent saidsection filled with gas to the surface of said body of water.
 20. Amethod as defined by claim 19 wherein the pumping of water from Said endof said pipeline at the lower elevation and the introduction of gas intosaid end of said pipeline at the higher elevation are continued untilsaid pipeline has been substantially voided of water and said end ofsaid pipeline at said lower elevation is thereafter lifted to thesurface of said body of water.